Total Marrow and Lymphoid Irradiation, Fludarabine, and Melphalan Before Donor Stem Cell Transplant in Treating Participants With High-Risk Acute Leukemia or Myelodysplastic Syndrome

Inclusion Criteria: * Eligible patients with a histopathological confirmed diagnosis of hematologic malignancy in one of the following categories: * Acute myelogenous leukemia: * Patients with de novo or secondary disease in unfavorable risk group including poor risk cytogenetics according to National Comprehensive Cancer Network (NCCN) guidelines for AML i.e., monosomal karyotype, -5, 5q-, -7, 7q-, 11q23-non t (9;11), inv (3), t (3;3),t (6;9), t (9;22) and complex karyotypes (≥ 3 unrelated abnormalities), or all patient in intermediate risk groups accept patients with FLT3-NPM1+ disease * Patients with active disease * Patients with chemosensitive active disease * Acute lymphocytic leukemia: * Patients with de novo or secondary disease according to NCCN guidelines for ALL hypoploidy (\< 44 chromosomes); t (v;11q23): MLL rearranged; t (9;22) (q34;q11.2); complex cytogenetics (5 or more chromosomal abnormalities); high white blood cell (WBC) at diagnosis (≥ 30,000 for B lineage or ≥ 50,000 for T lineage); iAMP21loss of 13q, and abnormal 17p * Patients with active disease * Patients with chemosensitive active disease * Myelodysplastic syndrome in high-intermediate (int-2) and high risk categories * Patients ≥ 12 years and \< 55 years are also included if they are not candidates for myeloablative conditioning regimens due to comorbidities * Karnofsky or Lansky performance status of ≥ 70 * A pretreatment measured creatinine clearance (absolute va…